Relay Circuit Diagram

Relay Circuit Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable relay circuit diagram: download the diagram as SVG or open it and print to paper or PDF.

A complete reference for relay circuit diagrams covering electromagnetic relay operation, normally open and normally closed contacts, coil ratings, and latching configurations.

A relay is an electrically operated switch. It uses an electromagnetic coil to actuate one or more mechanical contacts, allowing a low-power control signal to switch a higher-power load circuit. The separation between the coil circuit (control side) and the contact circuit (load side) is the fundamental advantage of a relay — it provides electrical isolation and allows a small signal from a microcontroller, PLC output, or sensor to switch mains voltage loads, motors, or high-current DC circuits.

A basic relay consists of an electromagnet coil, an armature, a return spring, and one or more sets of contacts. When current flows through the coil, the resulting magnetic field attracts the armature, which moves the contacts from their resting (de-energised) position to their energised position. When coil current ceases, the spring returns the armature and contacts to their resting state.

Contacts are described by their resting state: normally open (NO) contacts are open (circuit broken) when the relay coil is de-energised and close when energised. Normally closed (NC) contacts are closed (circuit made) when de-energised and open when energised. A change-over (CO) or single-pole double-throw (SPDT) contact has both NO and NC positions on one common (COM) terminal. A relay with multiple contact sets is described by its pole and throw count — a DPDT relay has two change-over contact sets.

Relay coil ratings specify the operating voltage and the minimum pull-in voltage and maximum drop-out voltage. A 12V DC relay coil typically pulls in at approximately 9V and drops out at approximately 3V. Coil resistance determines the holding current — for example, a 12V relay coil with 360 Ω resistance draws approximately 33 mA.

Inductive coils generate voltage spikes when switched off. In DC circuits, a flyback (freewheeling) diode is connected in reverse bias across the relay coil to absorb this spike and protect the switching transistor or output module. In AC circuits, an RC snubber serves the same function.

Time-delay relays, latching relays, and solid-state relays (SSRs) are specialised types. Latching relays require only a momentary pulse to set or reset the contact position and hold it without continuous coil power. SSRs use semiconductor switching (triac or SCR for AC, MOSFET for DC) instead of mechanical contacts, offering silent operation and unlimited switching cycles.

A relay circuit diagram shows how an electromagnetic relay is wired into a control and load circuit: the coil side (pins 85 and 86 on an ISO mini-relay) is driven by a low-current control signal or switch, while the contact side (pins 30, 87, and 87a) switches the higher-current load. The diagram typically labels the normally-open (NO), normally-closed (NC), and common (COM) contacts, the flyback diode across the coil to suppress inductive spikes, and any current-limiting resistors. Understanding both sides of the relay and their relationship is essential for safe design. Draw and customise your own relay circuit diagram free in the online editor.

How to wire relay circuit diagram

Identify relay specifications from the datasheet Before wiring, verify: coil voltage and type (AC or DC), coil resistance or current draw, contact configuration (SPST, SPDT, DPDT etc.), contact voltage and current rating (AC and DC separately listed), and pull-in/drop-out voltage thresholds. A relay operated from a 12V DC supply needs a contact rating that meets or exceeds the load voltage and current.
Connect the control (coil) circuit Connect the relay coil terminals to the control voltage source — typically a switch, PLC output, transistor, or microcontroller output. For DC coils, polarity matters only if a flyback diode is included. Connect the flyback diode in reverse bias across the coil (cathode to positive supply, anode to negative). For AC coils, either terminal can be live or neutral. Ensure the coil supply voltage matches the relay's rated coil voltage within ±10%.
Connect the load (contact) circuit Identify the contact terminals on the relay — typically COM, NO, and NC. Connect the load supply to the COM terminal. Connect the load device to NO for energised-to-activate logic, or to NC for de-energise-to-activate (fail-safe) logic. The load circuit is completely isolated from the coil circuit and can be a different voltage or AC/DC type.
Protect the switching element driving the coil If the coil is driven by a transistor or logic output, calculate whether the coil current is within the driving element's output current limit. Most microcontroller I/O pins are limited to 20–40 mA per pin — a relay coil drawing more than this requires an intermediate transistor driver stage. Verify the transistor's collector current and voltage ratings exceed the coil requirements.
Test coil energisation and contact switching Apply coil voltage and measure voltage at the coil terminals — confirm it is at the relay's rated value. Listen for the relay click (mechanical contact switching). Measure continuity across NO contacts — expect a closed circuit when energised. Measure across NC contacts — expect open. De-energise and verify the states reverse. This confirms correct wiring before connecting the actual load.
Connect and test under load Connect the load circuit. Apply coil voltage and verify the load device energises. De-energise the coil and verify the load de-energises. Measure contact voltage drop under load — excessive drop (more than 0.5V for DC loads) indicates a dirty or worn contact that may need cleaning or replacement. Check for relay chatter, which indicates coil voltage is marginal or the control signal is noisy.

Specifications

Typical coil voltage (industrial relay)	12V DC, 24V DC (control applications); 230V AC (mains-driven)
Relay pull-in voltage (typical)	75–80% of rated coil voltage
Relay drop-out voltage (typical)	10–30% of rated coil voltage
Contact rating (typical SPDT relay)	10 A at 250V AC; 10 A at 30V DC (verify per specific relay datasheet)
Mechanical life (electromechanical relay)	10 million operations (typical, no load)
Electrical life at rated load	100 000 operations (typical; derate for inductive or motor loads)
Operate time (typical electromechanical relay)	5–15 ms

Safety warnings

Relay contacts switching mains voltage (230V or 120V AC) carry lethal potential. Ensure the relay contact rating meets or exceeds the load voltage, current, and power factor. Do not exceed the rated contact current — relay contact failure under overcurrent can cause arcing, welding of contacts, and fire.
All mains-voltage relay wiring must comply with applicable electrical codes — NEC/NFPA 70 (USA), BS 7671 (UK), IEC 60364, or the applicable national standard. Mains wiring within relay panels must be segregated from low-voltage control wiring.
Always fit a flyback diode across DC relay coils driven from semiconductor outputs. Omitting this causes back-EMF spikes that can permanently damage microcontrollers, PLC modules, and transistors.
A solid-state relay (SSR) can fail in a short-circuit (closed) condition, leaving the load permanently energised. Safety-critical applications using SSRs must include additional independent isolation means or use mechanical relays with positive opening contacts.
Ensure the relay coil voltage matches the control supply. Applying higher than rated voltage to a relay coil causes overheating and coil failure. Applying significantly lower voltage results in unreliable pull-in or chatter.

Tools needed

Digital multimeter (voltage, resistance, continuity)
Non-contact voltage tester (for mains voltage circuits)
Oscilloscope (optional, for observing contact bounce and back-EMF spikes)
Soldering iron and solder (for PCB-mount relay installations)
Wire strippers and crimping tool (for panel wiring)
Logic probe or test lamp (for control signal verification)

Common mistakes

Omitting the flyback diode across a DC relay coil driven from a transistor or microcontroller output — back-EMF destroys the driving semiconductor on the first switching cycle.
Exceeding the relay contact current rating under inductive or motor loads — motor starting current can be 5–7× running current and must not exceed the contact's inrush rating.
Using NO contacts where NC contacts should be used for fail-safe applications — if the relay coil loses power, an NO-contact interlock opens the safety circuit as intended, but if the load must remain OFF on failure, NC contacts at the load circuit are required.
Driving the relay coil directly from a microcontroller I/O pin without a transistor driver when the coil current exceeds the pin's output limit — causes the pin to saturate, the relay to chatter, and potential damage to the microcontroller.
Mixing AC and DC contact ratings — a relay contact rated at 10 A AC may only be rated at 3 A DC due to the different arc-quenching characteristics. Always check both AC and DC ratings separately.

Troubleshooting

Relay coil energises (click heard) but NO contacts do not make connection: Cause: Contacts are welded open from a previous overcurrent event, or the wrong contact terminals are being used Fix: Measure continuity across the NO terminals when energised. If open, the contact may be welded open or the wrong terminals are used — check the relay body diagram or datasheet for correct COM, NO, NC terminal locations. A relay with damaged contacts must be replaced.
Relay chatters rapidly when coil is energised: Cause: Coil supply voltage is marginal (below pull-in voltage), or the control signal is noisy and toggling rapidly Fix: Measure coil voltage when chattering occurs — if it is below the relay's minimum pull-in voltage, the control circuit has insufficient drive or excessive impedance. Filter noisy control signals with a 100 nF decoupling capacitor close to the coil terminals. Increase driver capability or reduce coil circuit resistance.
Transistor driving relay coil fails repeatedly: Cause: Back-EMF spike from relay coil destruction is destroying the transistor — flyback diode is missing, reversed, or insufficient Fix: Verify flyback diode is present, correctly oriented (cathode to +V, anode to 0V across the coil), and is rated adequately. Check for a diode short using a multimeter diode-test mode. Replace the diode and transistor, and verify diode orientation before powering up.

Frequently asked questions

What is the difference between normally open (NO) and normally closed (NC) relay contacts?

Normally open (NO) contacts are open (no connection) when the relay coil is de-energised and close when the coil is energised. Normally closed (NC) contacts are closed (making connection) when the coil is de-energised and open when energised. The choice depends on fail-safe logic — a safety interlock should use NC contacts so that a coil failure or power loss causes the load to de-energise (fail-safe OFF).

Why do I need a flyback diode on a relay coil?

When current through the relay coil is interrupted, the collapsing magnetic field generates a voltage spike (back-EMF) that can reach hundreds of volts — well above the supply voltage. This spike damages transistors, microcontroller output pins, and PLC output modules. A flyback diode in reverse bias across the coil clamps this spike to approximately one diode drop (0.7V) above the supply voltage.

What is a latching relay and when is it used?

A latching (or bistable) relay has two stable contact states and holds its position after the energising pulse is removed — no continuous coil power is needed. It is used where the relay must retain its state during power interruptions, or where minimising power consumption is important. It requires a separate SET and RESET coil or pulse to change state.

Can I switch AC loads with a DC relay?

Yes, if the relay's contact rating includes AC voltage and current at the required level. The coil can be DC while the contacts switch AC — the two circuits are electrically isolated. Always check that the contact AC voltage and current ratings on the relay datasheet meet or exceed the load requirements. DC contact ratings are typically more restrictive than AC ratings due to arc quenching differences.

What is a solid-state relay (SSR) and how does it differ from an electromechanical relay?

A solid-state relay (SSR) uses semiconductor switching devices (triacs, SCRs for AC; MOSFETs for DC) rather than mechanical contacts. SSRs have no moving parts, operate silently, switch much faster, and have essentially unlimited switching cycles. However, they have a small voltage drop across the output (1–2V for triacs) causing heat dissipation, and they can fail short-circuit rather than open-circuit, which changes fail-safe analysis.

How do you explain a relay circuit diagram?

A relay circuit diagram is divided into two electrically isolated sides. The coil circuit (low-voltage side) provides current through the coil, creating a magnetic field that pulls the armature and closes (or opens) the contacts. The contact circuit (high-voltage or high-current side) carries the load — motor, lamp, solenoid — that you want to switch. A flyback or snubber diode is usually shown across the coil to protect the driving transistor or MCU pin from the inductive voltage spike when the coil de-energises.